The Town’s risk is buying electricity at a contracted rate for 20 years. At the beginning of the contract, electricity from the array is estimated to be about $0.07 less per kilowatt-hour. For the utility rate (NStar and generator) to be less than the array would require a sustained reduction in their pricing that has not happened since the 1960s.

The Town’s risk is buying electricity at a market (variable) rate for 20 years. Staying with the market rate is not a good deal when it exceeds the contracted price. That is the case at the beginning with the contracted price estimated at $0.07 per kWh less. From there, you assess risk of staying with the market rate by estimating the market price for future years and comparing it with the contract.